Regret accepting key delivery (6)
I believe the answer is:
rescue
'delivery' is the definition.
('rescue' can be a synonym of 'delivery')
'regret accepting key' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'accepting' is an insertion indicator.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'rue' enclosing 'esc' is 'RESCUE'.
(Other definitions for rescue that I've seen before include "Deliver from danger" , "liberation" , "Free from danger or captivity" , "delivery" , "Save - deliver" .)